[LyX/master] Missing include according to VC19

[Richard Kimberly Heck]

commit 91265c1f46ae9d326a58ec193518eee094f02ba7
Author: Richard Kimberly Heck <rikiheck at lyx.org>
Date:   Mon May 4 14:19:29 2020 -0400

    Missing include according to VC19
---
 src/mathed/InsetMathDecoration.cpp |    1 +
 1 files changed, 1 insertions(+), 0 deletions(-)

diff --git a/src/mathed/InsetMathDecoration.cpp b/src/mathed/InsetMathDecoration.cpp
index 35ba155..afb8228 100644
--- a/src/mathed/InsetMathDecoration.cpp
+++ b/src/mathed/InsetMathDecoration.cpp
@@ -27,6 +27,7 @@
 #include "support/lassert.h"
 #include "support/lstrings.h"
 
+#include <algorithm>
 #include <ostream>
 
 using namespace lyx::support;